<!DOCTYPE html>
<html>
<body>
<form name="theForm">
  <select name="theSelect" style="width: 30em">
    <option>xxxxxxxxxx</option>
  </select>
</form>
<script>
  // Flush reflow
  document.body.offsetWidth;
  document.theForm.theSelect.style.width='auto';
</script>
</body>
</html>

  
